GTP cyclohydrolase I (GCH1) catalyzes the first and rate-limiting step in the synthesis of tetrahydrobiopterin, the essential cofactor for tyrosine hydroxylase and the production of dopamine (DA) within nigrostriatal DA (NSDA) neurons. A complete analysis of the cis-acting elements in the GCH1 proximal promoter necessary for basal and cAMP-dependent transcription is near and the goal of Aim 1 is to identify cognate binding proteins for the GC-box and determine their role in basal and cAMP-dependent GCH1 transcription. The trans-acting factors recruited by these c/selements are also important and the goal of Aim 2 is to forge a link between phosphorylation of C/EBPbeta and NF-Y and cAMP-dependent GCH1 transcription. Little is known about how NSDA neurons regulate GCH1 gene expression. The goal of Aim 3 is to understand the temporal changes in protein-promoter DNA interactions that take place during cAMP-dependent GCH1 transcription in NSDA neurons and to test the hypothesis that GCH1 transcription is negatively coupled to somatodendritic D2 autoreceptor tone. Heterozygous mutations in GCH1 can cause DOPA-responsive dystonia (DRD), an autosomal dominant disorder with partial penetrance that selectively decreases DA synthesis within NSDA neurons and presents in childhood as a dystonia and in adulthood as Parkinson's disease (PD). Half of DRD patients have no mutation in the GCH1 open reading frame and presumably have mutations in GCH1 gene regulatory regions. The conserved genomic cis-elements we have already described are therefore likely sites for mutations associated with GCH1 deficiency. Unaffected first-degree relatives of DRD patients are also known to have a 23-fold higher incidence of parkinsonism than do normal controls, suggesting a link between DRD, GCH1 and PD. With the hypothesis that background genetic variability in GCH1 may promote susceptibility to familial parkinsonism and idiopathic PD, we propose in Aim 4 to sequence and functionally characterize mutations in GCH1 proximal promoter and coding regions in familial parkinsonism. Because association mapping is potentially a more powerful strategy for identifying genetic variability additional studies in Aim 4 will assess genetic variability within the GCH1 gene in PD cases versus controls. The goal of Aim 5 is to determine whether genetic variability in the human GCH1 gene influences GCH1 transcription or GCH1 enzyme activity. We expect that this multidisciplinary approach will yield important new information on the role of GCH1 in NSDA neuron function and will lead to a new understanding of DRD and familial and idiopathic PD.